Very fine processing is progressing due to the development of recent semiconductor process techniques. Accordingly, the degree of integration of semiconductor integrated circuits is also increasing abruptly which leads to an increase in the circuit scale. Moreover, recently, in order to produce effects such that an increase in a memory and low power consumption, a logic circuit and a large-scale memory device are mounted together onto one semiconductor chip.
When large-scale system circuits are mounted onto one semiconductor chip, a small area of a system substrate and low power consumption at the time of mounting a semiconductor device in a system apparatus can be attained. However, from the viewpoint of the fabrication of a semiconductor integrated circuit, the transistor density per unit area increases, and the chip area also increases. Accordingly, the defect factor of the wafer increases thereby causing deterioration of yield of the semiconductor chip.
Therefore, in the case where a portion of the circuit on a semiconductor chip, for example, a portion of the memory circuit is defectively produced in the fabricating process, a redundant circuit having a function equivalent to the defective circuit is previously mounted onto the same semiconductor chip, and a part of the redundant circuit is replaced with the defective circuit by a switching circuit. Sometimes the defective circuit is completely replaced with the redundant circuit. As a result, the defective circuit is not used, therefore, the yield of the semiconductor chip is improved.
As a method of replacing the defective circuit with the redundant circuit, there exists a laser trimming method utilizing a fuse, for example. One terminal of a fuse is connected with a power supply of a semiconductor integrated circuit or a ground node, and the other terminal is connected with a repair signal generating circuit which generates a repair signal. This repair signal indicates whether or not a defective circuit is replaced.
In the laser trimming method, a semiconductor integrated circuit is first tested, and when a defective portion or defective block is identified, a fuse of the specified defective portion or block is cut by a laser trimming device. When the fuse is cut, the repair signal generating circuit is actuated so that a repair signal output from the repair signal generating circuit is inverted. The defective portion or block is replaced with a redundant circuit having a function equivalent to the defective portion or block based on the inverted repair signal so that the yield of the large-scale semiconductor integrated circuit is improved.
FIG. 5 is a diagram showing a configuration of a conventional repair signal generating circuit using a fuse. This repair signal generating circuit outputs a high level repair signal OUT when a fuse 103 is connected, and outputs a low level repair signal OUT when the fuse 103 is cut.
As shown in FIG. 5, when the fuse 103 is connected, that is, is not cut, since the grounded fuse 103 is connected with an input of an inverter 104, a low level signal is input into the inverter 104. Accordingly, the inverter 104 outputs a high level signal. The output of the inverter 104 is output as the repair signal OUT and also input into a gate of a p-channel transistor 102. When the high level signal from the inverter 104 is input into the gate of the p-channel transistor 102, the p-channel transistor 102 becomes OFF.
In this state, when the fuse 103 is cut by the laser trimming device or the like, the input side of the inverter 104 becomes a floating node which is electrically isolated. Here, when power is supplied to the semiconductor integrated circuit to which this repair signal generating circuit is mounted, one terminal of a capacitor 101 is dragged abruptly to a high level, that is, electric charge from the capacitor 101 is discharged, and an electric charge which corresponds to the discharge of the electric charge is again allocated and simultaneously the electric potential changes at the node of the inverter 104 on the input side.
When the node of the input side of the inverter 104 is at high level once, the inverter 104 outputs a low level signal, and the low level repair signal is input into the gate of the p-channel transistor 102 so that the p-channel transistor 102 becomes ON. When the p-channel transistor 102 is ON, the voltage level of a power supply Vcc, that is, the high level repair signal is input into the inverter 104, and thereafter the p-channel transistor 102 is maintained in the ON state so as to serve as a latching circuit in which the low level repair signal output from the inverter 104 is held.
As a result, in the case where the fuse 103 is not cut, the repair signal generating circuit outputs the high level repair signal to a not shown switching circuit, and in the case where the fuse 103 is cut, outputs the low level repair signal to the switching circuit.
However, in the above-mentioned conventional repair signal generating circuit, since capacitance of a capacitor 101 is raised by the cutting of the fuse 103, the voltage of the input side of the inverter 104 is not sufficiently raised depending on whether the repair signal generating circuit is fabricated well or poorly. Accordingly, there arises a problem that the cut state of the fuse 103 cannot be detected securely.
In addition, when the fuse 103 is not cut completely, a very weak electric current flows in the fuse 103, and the power consumption cannot be lowered.